Troubleshooting
2-65
Test 308.2 — Overload ohms
Type
Pass/Fail
Fault message
Overload OHMS
Description
The 9.6µA ohms source is switched in with the /OHMD line at U223 and U220. U232 pin 2 to
3 connects REFBUF to U233 pin 3. 7.2V appears across R277. (For a more detailed description
of the circuit setup, see test 304.1.) In this test, there is no path provided for current to flow out-
side of the ohms source. Current flow is downward through the parallel path of Q211 and Q212,
through CR213, VR204, and Q214 to common. U238 sees the open-circuit ohms voltage of
about 6V at pin 3. It is used in a unity-gain configuration, and its output, which is the COMPLI-
ANCE signal, goes to comparator U215. The 2M control line is pulled flow, turning on U232
pin 15 to 14 and switching R288 in parallel with R234. Along with R235, this divider places pin
5 of U215 at 4.5V. Since the COMPLIANCE line at 6V is more positive than 4.5V, the
/OVLD_OHM line is pulled low at about -15V. All of the OVLD lines are latched by U202, par-
allel loaded into U201, and serially input to the A/D board at J1026 pin 15. There is no A/D mea-
surement for this test. The pass/fail criteria is based on the value of the /OVLD_OHM bit.
Drawing reference
Analog Board; 2002-100
Components
U220, U223, U232, U233, and associated components.
Bit patterns
Bit pattern*
Register
—U400—
01111011
—U811—
00001101
—U224—
00010111
—U432—
10000000
—U810—
00000011
—U206—
01110000
—U203—
10101010
—U411—
11111011
—U809—
11100111
—U207—
11001111
—U221—
11100111
—U406—
00000100
AD_STB
MUX_STB
R1_STB
R2_STB
*Bits associated with register IC terminals as follows:
Q
Q
Q
Q
Q
Q
Q
Q
87654321
87654321
87654321
87654321
IC pins: Q8=11, Q7=12, Q6=13, Q5=14, Q4=7, Q3=6, Q2=5, Q1=4.
